You should read up on Proust's law, better known as the Law of Definite Proportions. This is a chemical law that defines your question more generally, on why the ratio of elements and ions are always fixed.
Basically, this compound Magnesium(II) Chloride is MgCl2 because it has the same number of protons, neutrons, and electrons all the way. This defines the properties of the compound or atom.
Centimeters are units of length.
Steel
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It would be none would it not
To answer this, we use Raoult's Law where the partial pressure is equal to the product of the fraction of the gas in the mixture and its total pressure. Also, we use Dalton's Law of Partial Pressure where the total pressure is equal to the sum of the partial pressure of the gases in the mixture.
Ptotal = 73.44 + 128.52 + 2.04 = 204 atm
201.96 = x (204)
x = 0.99